This invention relates generally to coating substrate materials and more specifically, to a method and apparatus for providing a smooth, uniform coating on a web of paper, paperboard, film or other substances.
Coating a web of paper is generally effected by the application of a fluid coating material to a moving web. The material may be composed of a solid constituent suspended in a liquid carrier or vehicle. In one example, a pigment composition such as clay, calcium carbonate, aluminum powder, together, with an adhesive component such as starch, latex, casein, vegetable protein or equivalent are suspended in an aqueous vehicle.
The quality of coating on the paper depends upon many factors, one important factor being how the material is applied. The application of the coating material should preferably result in a coating that is continuous and uniform across the web. The coating material, varying in viscosity and solids content according to desired use, has previously been applied to the moving web in several ways.
One method previously employed was to feed the coating material to applicator rolls that applied the material directly onto the moving web. While the use of applicator rolls would initially yield a finished uniform coating across the web, as the process continued there occurred an eventual build up of coating material, dirt and other foreign substances on the rolls themselves that caused scratches and other imperfections in the coating. Direct application by rollers also created forces which imbedded the coating material into the web, and other drawbacks.
In an attempt to avoid these and other problems, the art developed a coating process in which the coating was jetted directly onto the moving web with a die fountain nozzle. While this process overcomes some of the limitations of roll applicators, die fountain nozzles require elaborate hydraulic mechanisms to create a uniform jet and, for some coating materials, elaborate coating/air separation equipment. Moreover, such process also requires special filtering apparatus and has a limited color viscosity and solids ranges.
Another important consideration in any coating application process is the velocity of the coating material as it is delivered from the carrying medium to the web surface. Because the web is moving during the process and because the coating material must be applied to the full width of the moving web, it is essential that the coating material be delivered with a uniform velocity across the entire width of the web. A jet from a fountain nozzle has a tendency to have less than a uniform velocity profile in the cross machine direction because of the inherent engineering problems in the equipment used to form the free steady jet fed from a pipe.
In addition to the objective of achieving a continuous, uniform coating on the web, the cost of achieving this goal, and the availability of an apparatus to handle a wide range of color solids content and viscosity are also of importance. It is also highly desirable to accomplish the result with a simple apparatus.
It is the primary object of the invention to provide an apparatus for applying a color coating onto a paper web, which apparatus is simple in construction and economical to operate and which is capable of applying a continuous, uniform coating across the web.
It is another primary object of this invention to provide an economical process for coating a substrate wherein a continuous application of coating material is achieved by delivering the coating material at a uniform velocity as it leaves its carrying medium to a deflector bar and then to the substrate.
It is a further object of the invention to provide an apparatus and process for applying color coating material on a web of paper by generating a continuous free standing jet curtain of coating material.
It is a further object of the invention to provide an apparatus and process for applying color coating material on a web of paper by generating a continuous free standing jet curtain and a coating of excellent quality over a wide range of solids content and viscosity of the coating material.